Okay, let's start at the beginning.
You had a 318 LA roller motor. Magnum 5.2s (318) were not introduced until 1992 ('93 for the 5.9). It has throttle body injection and a SBEC engine controller. Magnums had multi-point injection and an OBDI compliant computer.
I have no idea what year/spec 318 you put in, but chances are it doesn't have a fuel injection/computer friendly cam in it. Not a deal breaker, but the computer can be pretty finicky when it comes to camshaft centerlines; so if this was a reman intended for a carbureted vehicle it could be contributing to your issues. In addition, the distributor in the Mopar TBI not only controls the timing, but also acts as a crank sensor. There is no leeway when installing the slotted intermediate shaft/gear, it MUST be installed so the rotor is pointing to the #1 terminal when the timing marks are lined up to "0". No fudging- moving the plug wires around to get it to line up, that will mess up the crank sensor function.
To set your timing, it's recommended that you use a magnetic timing probe and connect your analyzer into your DRB diagnostic connector- but since almost NOBODY has these anymore, you'll have to do it with a standard induction timing light. Get the engine warm, and hook up your timing light to the #1 plug wire- then disconnect your coolant temperature sender. This is important!! The computer adjusts your timing in part according to engine temperature, you need to adjust your base timing, not computer-modified timing. Fire the engine up, and set your timing according to your sticker/FSM. Then lock the distributor down and reconnect the temperature sensor. You'll be showing a fault code now because of the sensor being disconnected, but you can erase it if your mech has the proper equipment (ha!), or by disconnecting your battery for five minutes or so.
If not done this way, it will cause the symptoms you're describing: bogging down, low speed drivability, and maybe some high speed misfires. This would be similar to setting a standard ignition without disconnecting your vacuum advance or plugging the vac. port. The computer requires an accurate base timing and crank sensor reading or you'll never get it to run right.
Good luck, and try to find a mechanic that knows something- anything, about Mopars. This one doesn't.
